Author: Zoe Tolman, Assistant Archivist

While the flailing capabilities were obviously an essential part to get right on the Crab, other testing was in force at the same time to try and address other issues brought up by its predecessors. 

After comments on the Scorpion's lack of wire-cutting, efforts were renewed with the Crab. The first production Crab prototype carried on from work done on the Baron and took the form of side cutters at each end of the rotor. These were similar to circular saws whose teeth passed through an open-ended slot in a bar mounted on the side arm. The slot had notches on it to catch the wire and allow the teeth to shear it as they passed through.

The cutter block (pulled back to clear a jammed or damaged cutter).

This system could successfully cut through 30 yards of entangled barbed wire and two rows of trip Dannert wire (a form of concertina wire developed in Germany in the 1930s, which didn’t require the usual vertical posts for attachment as it was largely self-supporting). Although wire would inevitably wind on the rotors, it was not enough at this level to impede their performance. It was therefore mainly satisfactory, but the ends of flails did occasionally also get sliced off by the wire cutters, reducing the efficiency of the flailing.

Unfortunately, with the progression of chain designs, this flaw became worse. Instead of being sliced off, the ends of flail chains actually became stuck in the wire cutters. This in turn stalled the entire rotor (and with it often the tank). The rotor also couldn't be restarted without members of the crew getting out of the tank to free the jammed flail, which is obviously a very dangerous prospect. As a first attempt to fix this, the end four chains on both sides of the rotor were reverted to the previous type, which didn’t jam. However, this meant that the flails dealing with mines directly in front of the tracks (incredibly important ones) were of lower quality/efficiency than the rest, which was undesirable at best.

In April 1944, they decided to address the wire-cutter design instead. They created a version that would still gather up wire to be cut, but would reject flails before they could become jammed. The first adjustment was to increase the pitch of the notches to make it harder for flails to access. This successfully led to no flails being jammed but there was no real wire being cut either. The rotating parts were now simply pushing the wire over the notches in the fixed section instead of catching and shearing it. The fixed aspects were, therefore, fitted with a tongue extending vertically downwards, in order to better guide the wire into the notches. This led to a vastly increased performance, as can be seen in the two images below. The fixed wire cutters were put into production in October 1944.

The Crab rotor after cutting through a triple Dannert and double apron fence, with original cutters (left) and the modified version (right).

The main improvement in the Crab, however, came with the arms. These had been through a great deal of development: from completely fixed, to adjustable (manually), to adjustable by hydraulics (as on the Crab I). Yet with all these, the rotor still functioned in fixed positions. This meant that it had a tendency to either miss mines or simply stall when it was travelling through undulating ground. It was proposed to allow the arm to pivot freely and counterbalance the rotor so that it instead followed the contours of the ground. When the rotor was too close to the ground, the force from the chains striking it would be large and so the rotor pushed back up; when the rotor was too far away from the ground, this force would lessen and so the rotor would fall back down.

The Baron had actually tried a contouring system when it was being tested in 1942, but in a much more manual sense. The hydraulics they used were not powerful enough to raise the rotor quickly enough to properly maintain a constant height above the ground. In December 1943, the principle was renewed on the Marquis and, again, the response of the system was deemed "so slow as to be quite useless." However, upon disconnecting the hydraulic rams on either side, the arms became much freer, and the response was then considered "perfectly instantaneous."

Trials started on an experimental Crab prototype in January 1944. Concern then shifted to potential damage to the arms caused by the explosive movements. Thankfully, tests done on a Mk IV mine showed that, although the rotor rose to the full height of 7'9", no damage was done. The placement of the counterweights was also very carefully tested to ensure that the cross tubes connecting the side arms would not twist. Initially this was spread out unevenly to reflect the differences of the arms (only one side was driven and so needed less weight to balance the forces from the explosions) but subsequent testing soon found that all of the counterweight could be placed on one side with no noticeable distortion taking place. They also found that they could safely remove one hydraulic ram entirely, but then reattach the other ram to still be functional but not dampen the required reaction as had first been found.

Trials with the contouring Crab showed a mine detonating efficiency over undulating ground of 90%, compared to 65% over the same ground with a fixed rotor. It did still retain a fixed stop to prevent the arm falling below 4'3" (primarily intended for wire cutting mode) and a further stop to prevent it dropping below 2'7". Both of these could be controlled from within the tank. Such success meant that the experimental prototype was upgraded to a production one. The Crab II was born. It was approved for production at the end of May 1944, at a time when the original order for 300 Crabs had been increased to a total of 689 for the year.

The Crab II.

However, it was then found that the rotor on the Crab II would ride low when going uphill and high when going downhill, due to the placement of the pivot in relation to the centre of gravity of the rotor and assembly. This effectively meant that the counterweight was further away from the pivot when going uphill and so more force was required to lift the rotor against it, and vice versa on the downwards slopes. This also impacted the rotor speed and torque. Initial plans were to use a second Sherman as a boost uphill and so reduce the strain on the Crab II’s engines, which were trying to make up for the drop in speed, but this was both difficult to do and not very effective.

They moved on instead to fitting an auxiliary gearbox between the engine and the flail. This allowed an increase in torque which saw improvements in flailing in general. It also meant they could adjust for the difficulties with the inclines without impacting the overall speed. An additional advantage of this system was also noted from reports of the Crabs at Le Havre in the Normandy Campaign. In a mine sweeping operation over clay soil after very heavy rain, the engine rpm of the Crab I dropped substantially and 40 of them were blown up on mines. Had they been fitted with these auxiliary gearboxes, they could have maintained their rotor speed and with it the mine detonating efficiency. As such, the gearbox was considered for both Crab I and Crab IIs and put into place by February 1945.

Front view of the Crab II turret.

During operations in France and the Low Countries, Crabs were extensively used. The average distance they were required to flail at any one time was 600-700 yards, although they were usually ordered to flail between certain fixed lines rather than through definitively known minefields. Nevertheless, in the first four months of their operations, each squadron of Crabs blew on average 440 mines. Only 15 vehicles were damaged beyond repair by mines. All the work from the very first flail tank contributed in some way to their success and they are an excellent demonstration of the importance of continued experimentation and improvement.

Information and images in this article are taken from E:03.0422.05 and E:05.0177.01.

Article published in The Craftsman, December 2023.